Powered mobile liquefied gas carriers

ABSTRACT

The gas produced by evaporation from the insulated vessels is separated into two component flows. A first component flow is compressed and while being cooled and liquified is used to transfer heat to the second component flow. The heated second component flow is then used as an energy source for driving the carrier, such as a ship. The reliquified flow is returned to the insulated vessel.

0 United States Patent U 1 [1 I1 3,864,918

Lorenz Feb. 11, 1975 [5 POWERED MOBILE LIQUEFIEDGAS 2,938,359 5/1960Cobb. Jr. et al. 62/50 x CARRIERS 2,940,268 6/l960 Morrison 62/73,229.472 l/l966 Beers 62/50 X [75] In entor: M chael Loren Ham rg,3,733,838 5/1973 Delahunty 62/54 Germany 3,766,734 l0/l973 Jones 62/5] X[73] Assignee: Sulzer Brothers, Ltd., Winterthur,

Switzerland Primary E.\'aminerMeyer Perlin F} d: M 7 7 AssistantExaminer-Ronald C. Capossela [22] 8y 1 l9 3 Attorney, Agent, orFirm-Kenyon & Kenyon Reilly PP 361,032 Carr 8L Chapin [30] ForeignApplication Priority Data May 27, 1972 Germany 2225882 [571 ABSTRACTJune 10, i972 Germany 2228382 The gas produced by evaporation from theinsulated June 21, 1972 Germany 2230263 vessels is separated into twocomponent flows A first component flow is compressed and while beingcooled [52] us. Cl 2 J 6 3436 and liquified is used to transfer heat tothe SCond [5]} l t Cl Folk 25/08 component flow. The heated secondcomponent flow [58] g i 240 243 is then used as an energy source fordriving the carrier, such as a ship. The reliquified flowis returned to62/52, 54, 60/3946, 651, 671 the insulated vessel [56] 'g';f ::f 21Claims, 5 Drawing Figures l,808,439 6/1931 Serriades 60/671 CID . 1POWERED MOBILE LIQUEFIED GAS CARRIERS This invention relates to poweredmobile liquefied gas carriers, particularly waterborne vessels such asmarine tankers, in which the liquefied gas is contained in at least oneinsulated vessel at the appropriate low temperature and substantiallynormal pressure, any gas produced by evaporation being collected andbeing supplied as an energy source to a combustion operation for drivingthe carrier. The invention also relates to methods of operating suchliquefied gas carriers.

When liquefied natural gas, methane or some similar material with a lowboiling point, is transported it is not possible to prevent the constantingress of heat from the exterior into the vessels despite goodinsulation and to prevent liquefied gas being evaporated by this heat.Efforts are made to minimise the economical losses resulting fromevaporation of the charge. This applies more particularly to thetransport of liquefied gases by sea. Gas produced by evaporation duringsuch transport is collected and is used as an energy source for theships propulsion system.

The result of previous methods are however unsatisfactory for reasonsexplained below. Despite the complex insulation with which the tanks ofthe liquefied gas tankers are provided the evaporation losses are stillbetween 0.20 to 0.35 percent of the total load per day, depending on thesize of the ship. Given a common standard size of ship which has cargospace for 125,000 m of liquefied natural gas, this means that theaverage daily loss of gas by evaporation amounts'to approximately 300 mcorresponding'to 178,000 m of gas at NTP per day. According to a processwhich in practice is.used almost exclusively, this amount of gas,representing an unavoidable cargo loss, is supplied as fuel to the mainpropulsion plant of the ship. To this end, it is necessary to extractthe gas from the tanks, to compress it and to heat it at least toambient temperature. The amount of evaporation stated in the previousnumerical example corresponds to an output of 29,700 metric shaft hpwhen the gas is burnt in a modern ships boiler plant. Since regulationsdo not permit gas to be used as the sole fuel and the maximum proportionmay not exceed 85 to 90 percent of the fuel such a ship must at presentbe provided with a propulsion plant which has a minimum rating of 33,000shaft hp.

The unavoidable evaporation losses also involve a reduction of theactually available transportation space because the evaporation lossesmust be allowed for during travelling. Moreover, a certain quantity ofliquefied gas must remain in the cargo tanks when the ship travels emptyso that the tanks may be constantly maintained at the specified lowtemperature so that the effective space is still further reduced. Owingto this reduction of the cargo space and because of the high cost of theliquefied gas thus transported, it follows that the solution adoptedhitherto of utilising the evaporation losses for heating the ship'sboilers or the like is economically unsatisfactory.

Conventional re-liquefying methods and plants have not so far beenemployed because such known methods and equipment involve high capitalcosts and call for large quantities of energy for operating them.

According to one aspect of the present invention, in a method ofoperating a powered mobile liquefied gas carrier in which the liquefiedgas is contained in at least one insulated vessel at the appropriate lowtemperature and substantially normal pressure, gas produced byevaporation is collected and is divided into two part flows, a firstpart flow being compressed in itself and, while being cooled andliquefied, transferring heat to the second part, the re-liquefied gas ofthe first part flow being expanded and returned to the vessel and theheated second part flow being supplied as an energy source for drivingthe carrier.

Accordingly, the basic idea of the invention is to divide the gasproduced by evaporation into two part flows of which one is used topreheat the other while requiring only a small amount of energy forcompression, heat dissipation being regulated so as to achieve renewedliquefaction of the previously compressed part flow.

By contrast to the previous methods in which the gas produced byevaporation and to be supplied for combustion is first compressed andthen heated, additional energy being required for both operations, theinvention proposes that the part flow to be used in the propulsion plantis first heated by the energy which is supplied for compressing the partflow that is to be reliquefied and is then compressed for furtherutilisation. This results in a particularly advantageous energyutilisation, the effective evaporation losses being also substantiallyreduced.

To cool the first part flow which is to be returned it is possible forthe second part flow to be utilised in a lower temperature range of thefirst part flow. The entire amount of gas produced by evaporation may bedivided into the two part flows in a ratio controlled by reliquefaction.

Preferably the first part flow is compressed substantially adiabaticallyby utilising energy derived from the drive of the carrier but the energymay be derived from an auxiliary source of power.

The second part flow, i.e., the part flow which is to be supplied to thepropulsion system of the gas carrier, may be compressed after it hasbeen heated by the first part flow, i.e., the part flow which is to bere-liquefied. The first and second part flows may be conducted incountercurrent for the said transfer of heat. The reliquefied gas may besubjected to after-cooling. In a modified method, in order to improvethe efficiency still further, the entire collected gas may be utilisedfor cooling the first part flow prior to division into the two partflows. The first part flow which is to be supplied to the compressortherefore has a higher temperature than in the first described method.The latter method has the advantage of reducing the amount of equipmentrequired. It is possible to use simpler and less expensive compressorsfor the part flow which is to be returned to the vessel and it is alsopossible to utilise heat exchangers which are smaller. In addition topermitting the use of less expensive apparatus, the modified method alsoleads to better utilisation of energy thus providing an overall economicimprovement. The compressor may operate with the part flow to bereturned at an inlet temperature which is approximately 40C higher thanin the first mentioned method.

Preferably cooling is performed with the collected gas in thelow-temperature range and the part flow which is to be supplied forcombustion is used for cooling thepart flow which is conducted throughthe compressor'and is to be returned to the vessel in the highertemperature range. The part flow of the collected gas to be supplied forcombustion may be branched off at a temperature level which correspondssubstantially to the condensation temperature.

The methods according to the invention may be relatively easily andsimply controlled. The ratio between the part flows fluctuates onlywithin narrow limits in normal operation. A three-way valve, which iscontrollable by the condensation pressure and is disposed at the pointof division into the branch ducts may be used to provide the maincontrol.

According to another aspect of the present invention, a mobile liquefiedgas carrier has means for utilising evaporated liquefied gas to drivethe carrier, an insulated vessel to contain liquefied gas, a gasdelivery duct leading from the vessel and dividing into branch ducts,the first branch duct leading through a compressor, one side of a heatexchanger means and an expansion element and back to the insulatingvessel, and the second branch duct leading through the other side of theheat exchanger means to the drive means.

The invention may be carried into practice in various ways but twosystems of operating a liquefied gas carrying ship and their mode ofoperation in accordance with the invention, together with a previouslyproposed system, will now be described with reference to theaccompanying drawings, in which:

FIG. 1 shows a ship in simplified form;

FIG. 2 is a diagram of a previously proposed system for dealing withevaporated gas;

FIG. 3 is a diagram of a system operating in accordance with the presentinvention;

FIG. 4 is a graph in which the pressure and enthalpy of the gas/liquidas it passes through the system shown in FIG. 3 are plotted; and

FIG. 5 is a diagram of a modified system.

FIG. 1 shows in simplified form a ship having a number of insulatedvessels 12, 14 which, in this case, are of spherical shape and containliquefied natural gas. Other shapes are possible and are commonly used.The insulation of the insulated vessels is constructed in the usual wayso that the evaporation loss resulting from the action of the heat ofwater and air on the vessels is reduced to the lowest level which iseconomically justifiable. The gas nevertheless generated due toevaporation of the liquefied gas is collected by a duct 18 whichcommunicates through connections 18a, 18b with the insulating vesselsl2, 14, the gas being supplied to the ships propulsion apparatus 16. Are-liquefying device 20 is provided into which the duct 18 leads andfrom which a duct 24 extends to a device in the ships propulsionapparatus 16 to deliver a part flow of the evaporated gas for combustionin the apparatus 16. In this way, the gas is burnt to yield thermalenergy. A duct 22 also extends from the device 20 to the vessels 12, 14in order to return re-liquefied gas via connections 22a, 2212 into thevessels. Where there is a number of vessels it is not necessary for allthe vessels to be connected to the return duct 22. Since only part ofthe gas produced as a result of evaporation is again liquefied it issufficient to provide connections for a corresponding maximumre-liquefaction flow.

To illustrate the advantages of the invention, FIG. 2 shows a previouslyproposed system by means of which gas produced by the evaporation ofliquefied gas was utilised for combustion. In this device the collectedgas is supplied through a duct 100 to a compressor 102 the output ofwhich is connected through a duct 104 to a heat exchanger 106. The gasdischarged from the heat exchanger is fed through a duct 108 into acombustion device. At the inlet to the compressor 102 the gas has atemperature of approximately -l50C and a pressure p 1 atm abs. At theoutput of the compressor t -l25C, p 1.7 atm abs. After leaving the heatexchanger, t= +20C, p 1.7 atm abs.

The heat exchanger 106 is operated with a glycolwater mixture which mustbe correspondingly preheated. To this end, a heat exchanger 112 isprovided which is supplied with steam via a duct 110. The exhaust steamfrom the heat exchanger 112 is discharged via a duct 114. Theglycol-water mixture heated by the steam passes from the heat exchanger112 via a duct 116 to the heat exchanger 106. Convection in this case isinsufficient to ensure circulation and for this reason a pump isprovided for the glycol-water mixture circulation. The partiallyevaporated glycol-water mixture passes from the heat exchanger 106 via aduct 119 into a glycol-water mixture storage tank 118. A by-passconnection 117 is provided between the duct 116 and the duct 119. Thestorage tank 118 communicates with the inlet of the pump 120 through aduct 121. The system is provided with valves for regulation purposes,these valves being controlled by devices designated by the letters TC independence on the temperatures which prevail in the various parts of thesystem.

Pressure control means 103 are provided for the compressor 102. A levelindicator LI is also provided for monitoring purposeson the tank 118 andis adapted to deliver a signal for controlling the system when the levelapproaches a maximum or minimum.

The explanation above of the previously proposed system indicates thaton the one hand it calls for a substantial amount of equipment and onthe other hand prepares the gas resulting from evaporation practicallyonly for combustion to which end additional substantial amounts ofenergy are required.

A re-liquifying device 20 constructed and operating in accordance withthe invention is shown in detail in FIG. 3. In this device, the duct 18leads to a controllable three-way valve 26 in which the entire incomingflow of gas is divided into two part flows. This division is performedat a defined, controlled ratio. One part flow is fed by the valve 26through a duct 28 to the inlet of a compressor 30 and compressed. Theoutlet of the compressor is connected through a duct 32 to a condenser34 which, together with a condensate collector and after-,cooler 36,,forms an integral structural unit operating as a heat exchange means.The gas which is heated and compressed by the compressor 30 is liquefiedin the unit 34, 36 after giving up heat to the gas which is to be burnt.To this end, the compressed gas is passed into heat exchange relationwith the other gas part flow to cool and reliquify the compressed partflow while heating the other part flow. After being cooled, theliquefied gas which is received by the collector may be returned by aduct 22 and an expansion valve 62 to the vessels 12, 14. The second,larger part flow flows from the valve 26 through a duct 40 to the gasduct system of the collector and aftercooler 36, illustrated insimplified form as a cooling coil, in counter-flow to the compressed gasflow so as to cool and reliquify the compressed gas flow. A duct 44 withwhich the duct 40 communicates through a by-pass 46 containing a valve,supplies the gas to a cooling coil 48 in the condenser 34. From therethe gas which has been substantially heated passes through a duct 50 tothe inlet of a com pressor 52 and is appropriately compressed thereinfor combustion. The gas then passes through a duct 24 to the combustionmeans of the propulsion system. The compressors 30, 52 are also used fordrawing the gas from the vessels l2, l4.

The system shown in FIG. 3 is provided with appropriate means forcontrolling the process in the individual sections. Pressure-dependentregulating devices are designated with the letters PC in FIG. 3 whileregulating devices which depend on the filling level are designated withthe letters LC. Pressure regulating means 54 are disposed between theduct 18 and the compressor 52 to ensure that the pressure in the vesselsl2, 14 remains constant. Pressure-dependent rotational speed regulatingmeans 56 are provided for the compressor 30. To divide the gas whichflows through the duct 18 into the part flows the valve 26 is controlledby the condensation pressure (compression pressure) in the duct 32 bymeans of a device 58.

As regards the collector 36 it is important that it is always filled toa minimum level and does not exceed a maximum level. A level controlsystem 60 which controls an expansion valve 62 in the return duct 22 isprovided to control this state.

The invention is further explained by reference to the graph shown inFIG. 4. The enthalpy i is plotted on the abscissa and the logarithm ofthe pressure log p is plotted on the ordinate. The curves labelled rhand "'1 show the changes in pressure in enthalpy in the first part flowpassing at a rate ril from the valve 26 along the duct and the secondpart flow passing at a rate rh from the valve 26 along the line 40, thereferenced points on the curves corresponding to the similarlyreferenced parts of the system shown in FIG. 3.

The following relationships must be observed in considering the graph:

Q effective for liquefaction ,5 I [2 (in I) Table 1 Point T in K p in mmabs 1' in kcal/kg l 123 1.0 127.7 2 339 40 232.4 3 186.5 40 79.5 4 I3340 19.8 5 112.5 1.06 19.8 2 a 300 0.95 218.8 3 a 350 1.7

Gas proportion in the condensate x 0.155

Ai =i i 232.4 19.8 212.6 kcal/kg Ai =.i i, 218.8 127.7 91.1 kcal/kg m lmtotal 91.1/212.6 91.1 130.3 30

The numerical example confirms that approximately one third of the gasyielded by evaporation may be reliquefied with apparatus of the sameorder of magnitude as that employed hitherto. The graph of FIG. 4 alsoshows that pressure and temperature are initially increased for the partflow that is to be liquefied. The temperature is then reduced atconstant pressure. liquefaction occurring at a defined point whichdepends on p and T. After further cooling the gas is expanded combinedwith further temperature reduction.

A modified re-liquifying device 20 constructed and operating inaccordance with the present invention is illustrated in FIG. 5. The duct18 leads to the three-way valve 26 from which a line 28 leads to a heatexchanger unit 65. The heat exchanger unit 65 comprises three parts,namely an outlet cooler 66, a condenser 67 and an inlet cooler 68. Allparts of the heat exchanger unit are preferably combined in onestructure. The duct 28 extends through the outlet cooler 66 as a coolingduct 69 which leads into the cooling duct 70 of the condenser. A coolingduct 72 in the inlet cooler 68 communicates with the cooling duct 70 viaa three-way valve 71. The stream of collected evaporated gas issubdivided into two part flows by the three-way valve 71. One part flowpasses from the valve 71 through the cooling duct 72 of the inlet coolerand a duct 73 to a compressor 74 where the gas of the part flow whichhas been heated in the meantime is compressed for combustion.

A part flow which is smaller than the part flow intended for combustionpasses from the valve 71 through a duct 40 into a compressor 75 in whichthe gas which has already been heated in the heat exchanger unit abovethe original temperature of the vessels 12, 14 is compressed while beingheated, substantially'adiabatically. The compressed gas which now has atemperature substantially higher than the original temperature of thevessels then passes through a duct 76 into the heat exchanger unit 65.In this unit the gas from the compressor 75 gives up heat bycounter-flow to the gas which passes to combustion and to the undividedstream of the collected gas. After pre-cooling in the inlet cooler 68,the gas is liquefied in the condenser 67 and is further cooled in theoutlet cooler 66. The liquefied cooled gas passes through an expansionvalve 77 into the duct 22 for return to the vessels 12 and 14.

To start the system a part flow is first branched off at the valve 26and passed via a duct 78 into the duct 40 and thence to the compressor75. The duct 78 passes through a heating device 79 which may beoperated, for example, with sea water and which replaces preheating inthe zones 66 and 67 during the starting phase. The device or system ischanged over after starting up so that none of the collected gas isbranched off at the valve 26 and the division is performed at the valve71.

The system incorporates various controllers which are shown as P or LCrespectively in the drawing. LC refers to a level controller disposed onthe outlet cooler to ensure that a defined liquid level is alwayspresent in the outlet cooler 66. The controller LC thereforecommunicates with a valve 77 which also has a volumetric controlfunction for expansion. Control devices are also provided at thecompressors 74, 75. For example the controller on the compressor 74 maycommunicate with the valve 26 and establish a relationship between thegas supplied from the vessels and the gas which is to be delivered forcombustion. The pressure at the outlet from the compressor 75 may berelated to the dividing ratio at the valve 71.

In the system shown in H6. the entire mass flow from the vessels 12 and14 gives up part of its refrigeration energy for condensation and finalcooling of the part flow which is branched off from the total flow afteremerging from the condenser. It is essential that the entire mass flowis utilised for cooling the return part flow over a substantial part ofthe negative temperature range.

What we claim is:

l. A method of operating a powered mobile liquefied gas carrier in whichliquefied gas is contained in at least one insulated vessel at theappropriate low temperature and substantially normal pressure, in whichmethod gas produced by evaporation is collected and is divided intofirst and second part flows, the first part flow being compressed initself and, while being cooled and liquefied, transferring heat to thesecond part flow, the reliquefied gas of the first part flow beingexpanded and returned to the vessel and the heated second part flowbeing supplied as an energy source for driving the carrier.

2. A method as claimed in claim 1 in which the second part flow isadditionally used for cooling the first part flow, which is to bereturned, in a lower temperature range of the first part flow.

3. A method as claimed in claim 1 in which the whole of the gas producedby evaporation is divided into the two part flows at a ratio controlledby the reliquefaction of the first part flow.

4. A method as claimed in claim 1 in which the first part flow iscompressed substantially adiabatically by utilising energy derived fromthe drive of the carrier.

5. A method as claimed in claim 1 in which the second part flow iscompressed after having been heated by the first part flow.

6. A method as claimed in claim 1 in which the first and second partflows are conducted in countercurrent for the said transfer of heat.

7. A method as claimed in claim 1 in which the reliquefied gas issubjected to final cooling.

8. A method as claimemd in claim 1 in which the entire collected gas isutilised for cooling the first part flow prior to division into two partflows.

9. A method as claimed in claim 3 in which cooling is performed with theentire collected gas in a low temperature range and the second part flowis utilised for cooling the first part flow conducted through thecompressor and returned to the vessel in a temperature range above thelow temperature range.

10. A method as claimed in claim 9 in which the second part flow isbranched off from the collected gas at a temperature level whichcorresponds substantially to the temperature of condensation.

11. A method as claimed in claim 9 in which the entire gas flow isdivided into the two part flows after passing through a final coolingzone and condensation zone for the first part flow.

12. A mobile liquefied gas carrier having means for utilising evaporatedliquefied gas to drive the carrier, an insulated vessel to containliquefied gas, a gas delivery duct leading from the vessel and dividinginto branch ducts, the first branch duct leading through a compressor,one side of a heat exchanger means and an expansion element and back tothe insulating vessel, and the second branch duct leading through theother side of the heat exchanger means to the drive means.

13. A gas carrier as claimed in claim 12 in which the heat exchangermeans includes a condenser for the flow in the first branch duct and acondensate receiver through both of which the gas delivery duct passes.

14. A gas carrier as claimed in claim 12 in which the heat exchangermeans comprises an inlet cooler, a condenser and an outlet cooler whichare connected in series in the first branch duct and are combined intoone structural unit.

15. A gas carrier as claimed in claim 12 in which the Heat, exchangermeans includes a condenser for the flow in the first branch duct and acondensate receiver through both of which the second duct passes.

16. A gas carrier as claimed in claim 15 in which the second branch ductextends from the heat exchange means via a compressor to the drivemeans.

17. A method of operating a propulsion system for a powered mobileliquified gas carrier having at least one insulated vessel containingliquified gas, said method comprising collecting gas generated due toevaporation in the vessel,

dividing the collected gas into two part flows,

compressing one of said part flows,

passing the compressed part flow into heat exchange relation with theother of said part flows to cool and re-liquify the compressed part flowwhile heat ing said other part flow,

returning said re-liquified part flow to one of the vessels, and

passing said heated part flow to the propulsion system for driving thecarrier.

18. A mobile liquid gas carrier having a propulsion apparatus fordriving the carrier,

at least one vessel for containing liquified natural gas,

a gas delivery duct communicating with said vessel to collect evaporatedgas from said vessel,

a re-liquifying device connected to said duct to receive the evaporatedgas for re-liquifying a part flow of the evaporated gas,

a first duct extending from said device to said vessel to return thepart flow of re-liquified gas from said device to said vessel, and

a second duct extending from said device to said propulsion apparatus todeliver a second part flow of evaporated gas from said device to saidpropulsion apparatus for combustion therein.

19. A mobile liquid gas carrier as set forth in claim 18 wherein saidre-liquifying device includes a controllable three-way valve connectedto said gas delivery duct; a heat exchanger unit having an outletcooler, condenser and inlet cooler; a first cooling duct in said outletcooler connected to said valve to receive the evaporated gas; a secondcooling duct in said condenser connected to said first cooling duct toreceive the evaporated gas therefrom; a second three-way valve connectedto said second cooling duct; 21 third cooling duct in said inlet coolerconnected to said secand valve to receive a part flow of evaporated gastherefrom; and a compressor connected to said second valve to receiveand compress a second part flow of the evaporated gas therefrom, saidcompressor being connected to said heat exchanger unit to deliver thecompressed second part flow thereto for counterflow over said coolingducts.

20. A mobile liquid gas carrier as set forth in claim 18 wherein saidre-liquifying device includes a controllable three-way valve connectedto said gas delivery duct, a compressor connected to said valve toreceive and compress a first part flow of the evaporated gas, a heatexchange means connected to said compressor to receive the compressedfirst part flow from said compressor, said heat exchange means beingconnected to said valve to receive a second part flow of the evaporatedgas for flow therethrough in 'countercurrent to the compressed firstpart flow to cool and re-liquify the compressed first part flow.

21. A mobile liquid gas carrier as set forth in claim 20 wherein saiddevice further includes a second compressor connected to said heatexchange means to receive and compress the second part flow and whereinsaid first duct connects to said heat exchange means to receive thecooled first part flow and said second duct connects to said secondcompressor to receive the

1. A method of operating a powered mobile liquefied gas carrier in whichliquefied gas is contained in at least one insulated vessel at theappropriate low temperature and substantially normal pressure, in whichmethod gas produced by evaporation is collected and is divided intofirst and second part flows, the first part flow being compressed initself and, while being cooled and liquefied, transferring heat to thesecond part flow, the re-liquefied gas of the first part flow beingexpanded and returned to the vessel and the heated second part flowbeing supplied as an energy source for driving the carrier.
 2. A methodas claimed in claim 1 in which the second part flow is additionally usedfor cooling the first part flow, which is to be returned, in a lowertemperature range of the first part flow.
 3. A method as claimed inclaim 1 in which the whole of the gas produced by evaporation is dividedinto the two part flows at a ratio controlled by the re-liquefaction ofthe first part flow.
 4. A mEthod as claimed in claim 1 in which thefirst part flow is compressed substantially adiabatically by utilisingenergy derived from the drive of the carrier.
 5. A method as claimed inclaim 1 in which the second part flow is compressed after having beenheated by the first part flow.
 6. A method as claimed in claim 1 inwhich the first and second part flows are conducted in countercurrentfor the said transfer of heat.
 7. A method as claimed in claim 1 inwhich the re-liquefied gas is subjected to final cooling.
 8. A method asclaimemd in claim 1 in which the entire collected gas is utilised forcooling the first part flow prior to division into two part flows.
 9. Amethod as claimed in claim 3 in which cooling is performed with theentire collected gas in a low temperature range and the second part flowis utilised for cooling the first part flow conducted through thecompressor and returned to the vessel in a temperature range above thelow temperature range.
 10. A method as claimed in claim 9 in which thesecond part flow is branched off from the collected gas at a temperaturelevel which corresponds substantially to the temperature ofcondensation.
 11. A method as claimed in claim 9 in which the entire gasflow is divided into the two part flows after passing through a finalcooling zone and condensation zone for the first part flow.
 12. A mobileliquefied gas carrier having means for utilising evaporated liquefiedgas to drive the carrier, an insulated vessel to contain liquefied gas,a gas delivery duct leading from the vessel and dividing into branchducts, the first branch duct leading through a compressor, one side of aheat exchanger means and an expansion element and back to the insulatingvessel, and the second branch duct leading through the other side of theheat exchanger means to the drive means.
 13. A gas carrier as claimed inclaim 12 in which the heat exchanger means includes a condenser for theflow in the first branch duct and a condensate receiver through both ofwhich the gas delivery duct passes.
 14. A gas carrier as claimed inclaim 12 in which the heat exchanger means comprises an inlet cooler, acondenser and an outlet cooler which are connected in series in thefirst branch duct and are combined into one structural unit.
 15. A gascarrier as claimed in claim 12 in which the heat exchanger meansincludes a condenser for the flow in the first branch duct and acondensate receiver through both of which the second duct passes.
 16. Agas carrier as claimed in claim 15 in which the second branch ductextends from the heat exchange means via a compressor to the drivemeans.
 17. A method of operating a propulsion system for a poweredmobile liquified gas carrier having at least one insulated vesselcontaining liquified gas, said method comprising collecting gasgenerated due to evaporation in the vessel, dividing the collected gasinto two part flows, compressing one of said part flows, passing thecompressed part flow into heat exchange relation with the other of saidpart flows to cool and re-liquify the compressed part flow while heatingsaid other part flow, returning said re-liquified part flow to one ofthe vessels, and passing said heated part flow to the propulsion systemfor driving the carrier.
 18. A mobile liquid gas carrier having apropulsion apparatus for driving the carrier, at least one vessel forcontaining liquified natural gas, a gas delivery duct communicating withsaid vessel to collect evaporated gas from said vessel, a re-liquifyingdevice connected to said duct to receive the evaporated gas forre-liquifying a part flow of the evaporated gas, a first duct extendingfrom said device to said vessel to return the part flow of re-liquifiedgas from said device to said vessel, and a second duct extending fromsaid device to said propulsion apparatus to deliver a second part flowof evaporated gas from said device to said pRopulsion apparatus forcombustion therein.
 19. A mobile liquid gas carrier as set forth inclaim 18 wherein said re-liquifying device includes a controllablethree-way valve connected to said gas delivery duct; a heat exchangerunit having an outlet cooler, condenser and inlet cooler; a firstcooling duct in said outlet cooler connected to said valve to receivethe evaporated gas; a second cooling duct in said condenser connected tosaid first cooling duct to receive the evaporated gas therefrom; asecond three-way valve connected to said second cooling duct; a thirdcooling duct in said inlet cooler connected to said second valve toreceive a part flow of evaporated gas therefrom; and a compressorconnected to said second valve to receive and compress a second partflow of the evaporated gas therefrom, said compressor being connected tosaid heat exchanger unit to deliver the compressed second part flowthereto for counterflow over said cooling ducts.
 20. A mobile liquid gascarrier as set forth in claim 18 wherein said re-liquifying deviceincludes a controllable three-way valve connected to said gas deliveryduct, a compressor connected to said valve to receive and compress afirst part flow of the evaporated gas, a heat exchange means connectedto said compressor to receive the compressed first part flow from saidcompressor, said heat exchange means being connected to said valve toreceive a second part flow of the evaporated gas for flow therethroughin countercurrent to the compressed first part flow to cool andre-liquify the compressed first part flow.
 21. A mobile liquid gascarrier as set forth in claim 20 wherein said device further includes asecond compressor connected to said heat exchange means to receive andcompress the second part flow and wherein said first duct connects tosaid heat exchange means to receive the cooled first part flow and saidsecond duct connects to said second compressor to receive the heatedsecond part flow.